This invention relates to refrigeration systems, and more specifically to a method and apparatus for improving the efficiency, reliability, and manufacturability of the compressor control system for an air conditioning unit.
Conventional refrigeration systems utilize a recirculating refrigerant for removing heat from a low temperature side of the refrigeration system and for discharging heat at a high temperature side of the refrigeration system. The work input necessary to operate the refrigeration system is provided by a motor driven compressor which receives low pressure gaseous refrigerant and compresses it to a high pressure. This high pressure gaseous refrigerant is supplied to a condenser where heat is removed from the gaseous refrigerant to condense it to a liquid. This liquid refrigerant is then supplied through an expansion valve to an evaporator wherein heat is transferred from a heat transfer fluid to the liquid refrigerant to evaporate the liquid refrigerant. The heat transfer fluid is thereby cooled and then used to cool a load, such as to cool a building. The evaporated refrigerant from the evaporator is returned to the compressor for recirculation through the refrigeration system. A control system directs the operation of the air conditioning unit.
Normally, the heat transfer fluid used in an evaporator of a conventional refrigeration system of the type described above is a liquid such as water. Usually, the liquid enters one end of the evaporator, is cooled as it flows through the evaporator, and then exits at another opposite end of the evaporator. It is highly desirable to maintain the heat transfer liquid flowing through the evaporator at a temperature above the freezing temperature of the heat transfer liquid. If the liquid is not maintained above its freezing temperature then the liquid may freeze in the evaporator thereby preventing proper operation of the refrigeration system and possibly damaging the evaporator. This is especially true if the heat transfer fluid is water because water increases in volume when changing state from a liquid to a solid.
The compressor electronic control system is designed to replace the electromechanical control systems of large commercial air conditioners having multiple compressor and unloaders that are cycled on and off to give multiple stages of capacity. The cycling is usually accomplished by a controller that cycles relays on and off in one predetermined sequence. The different loading sequences were accomplished by wiring to the step controller in different ways. The wiring is complicated and requires relays with both normally open and normally closed contacts.
Further, most large chillers are designed with two circuits in order to have a standby operation. Normally, a manual lead/lag switch will allow the operator to field change the compressor loading sequence to equalize the run hours on the lead compressors in each circuit. However, the lead/lag switch usually changes only which circuit will start first and after that the loading sequence is the same.
Still further, most chillers are designed with controls that maintain a constant leaving cooler water temperature. The water temperature is usually set at the temperature required at full load. However, at part load it may not be necessary to maintain the leaving water temperature of the full load set point, because the machine over-cools the water and is less efficient. Also, the cycle points for different stages of the compressors are based on a fixed temperature differential to be seen over the complete load range. As this load changes, however, this differential is excessive which results in inaccurate temperature control.
Further, most reciprocating water chillers can supply leaving chilled water within a temperature band of about 5.degree. F., with an additional droop of about 11/2 degrees. Fluctuating chilled water temperatures can cause air conditioning control problems, occasionally interfere with good humidity control, and it also can make people uncomfortable. It also can make precise control of industrial processes impossible. The fluctuations stem from controlling only from return water temperature to sense building load.
The present invention controls on leaving chilled water temperature but also senses return water temperature to anticipate building load changes. A microprocessor is programmed to measure both temperature differences and rate of change. From this information the microprocessor commands the compressors. The result is no droop and a better temperature control. Further, the leaving water temperature of the chiller is reset based on the return water temperature, and adjusted by a reset ratio means or potentiometer.
Ability to replace moving parts and flow switches in a control system and have a simple means to allow for the programming of a very sophisticated electronic control during the assembly of an air conditioning unit and in the field would represent a significant savings in the installation and maintenance of the unit and a higher operating efficiency of the unit. Thus, there exists a need for a method and apparatus which utilize electronic components to control the loading and unloading of compressors in an air conditioning unit and to automatically adjust the stage differential temperatures and reset of the leaving water temperature.